Field of the Invention
The present invention relates to a PWM rectifier for a motor drive, which controls a switching element using a PWM signal to convert three-phase AC power into DC power, and in particular relates to a PWM rectifier connected, through an electric storage device which can store DC power, to the DC side of an inverter which performs a power conversion between DC power and AC power being drive power or regenerative power of a motor.
Description of the Related Art
In a motor drive unit for driving motors used in machine tools, industrial machines, forming machines, injection molding machines, or various kinds of robots, DC power temporarily converted from AC power which is input from an AC power supply side is further converted to AC power. The converted AC power is used as drive power of a motor provided for each drive axis. The motor drive unit includes: a rectifier for outputting DC power by rectifying AC power supplied from an AC power supply side provided with a commercial three-phase AC power supply; and an inverter, being connected to a DC link at a DC side of the rectifier, for performing a bidirectional power conversion between DC power at the DC link and AC power being drive power or regenerative power of a motor. The motor drive unit controls the speed, torque, or position of a rotor of the motor connected to the AC side of the inverter.
In an acceleration or deceleration control of a motor with a motor drive unit, an output or regeneration with large amount of AC power is required to an AC power supply, which leads an occurrence of an electric power peak. Then, the power supply capacity on an AC power supply side from which power is supplied to the motor drive unit is generally designed taking the electric power peak which occurs at the time of acceleration and deceleration of the motor into consideration. However, according to the design in consideration of the electric power peak which occurs at the time of the acceleration and deceleration of the motor, a power supply capacity is inevitably large compared to a case in a simple design in consideration of the average power of a motor drive unit. Especially in a motor drive unit with many opportunities of quick accelerations and quick decelerations of a motor, a power supply capacity is much larger. An installation cost and operation cost increase as a power supply capacity becomes larger, and therefore it is preferable to reduce the power supply capacity.
In order to reduce a power supply capacity, a motor drive unit may be provided with a PWM (Pulse Width Modulation) rectifier which can perform a power running operation (conversion operation) converting AC power into DC power and a regenerative operation (inverse-conversion operation) inverting DC power into AC power, and an electric storage device which is connected in parallel with a DC side of the PWM rectifier and can store DC power. With this configuration, by adequately controlling the PWM rectifier to adjust an amount of each power conversion in a power running operation (conversion operation) converting AC power into DC power and a regenerative operation (inverse-conversion operation) inverting DC power into AC power performed by the PWM rectifier, it is possible to store regenerative power generated by a motor at the time of deceleration of the motor in an electric storage device, and to reuse the stored power at the time of acceleration of the motor, thereby reducing a power supply capacity.
FIG. 10 is a diagram illustrating a configuration of a common motor drive unit which includes a PWM rectifier. Hereinafter, one to which the same reference numeral is given in different drawings is a component having the same function. A motor drive unit 100 includes a PWM rectifier 10 which converts AC power from a commercial three-phase AC power supply (hereinafter, simply referred to as “AC power supply” in some cases) 4 into DC power, and an inverter 2 which converts the DC power output from the PWM rectifier 10 into the AC power with a desired frequency to be supplied as drive power of a motor 3, and which converts the AC power regenerated by the motor 3 into DC power. The motor drive unit 100 controls the motor 3 to adjust the speed, torque, or the position of a rotor of the motor 3 connected to the AC side of the inverter 2. The PWM rectifier 10 is connected to an AC reactor 5 on a three phase AC input side.
The inverters 2 are provided, the number of which is the same as the number of the motors 3, in order to individually supply the drive power to the respective motors 3 each provided for corresponding one of a plurality of drive axes to perform a drive control of the motors 3. Note that in the depicted example, it is assumed that the number of the motor 3 is one, and therefore the number of inverter 2 is also one in this case. In many cases, one PWM rectifier 10 is provided with respect to a plurality of inverters 2 for the purpose of reducing the cost and the occupancy space of the motor drive unit 100.
The PWM rectifier 10 is configured by a main circuit unit 11 including a bridge circuit formed with a switching element and a diode connected to the switching element in antiparallel, and a PWM rectifier control unit 12 which generates a PWM control signal for controlling a switching operation of the switching element in the main circuit unit 11. Although regeneration power occurs by the motor 3 when decelerating the motor 3 by a control of the motor drive unit 100, such regeneration power can be returned to the PWM rectifier 10 through the inverter 2. The PWM rectifier 10 can perform a regeneration operation (inverse-conversion operation), in which DC power is converted into AC power, with the control of the switching operation of the switching element in the PWM rectifier 10 by the PWM control signal, and can return the regenerative energy returned from the inverter 2 to the AC power supply 4 side.
The PWM rectifier control unit 12 in the PWM rectifier 10 generates the PWM control signal from an AC voltage value on the AC power supply 4 side detected by an AC voltage detection unit 21, an AC current value on the AC power supply 4 side detected by an AC current detection unit 22, and a DC voltage value at an electric storage device 6 detected by a DC voltage detection unit 23 (a DC voltage value across a DC link between the main circuit unit 11 in the PWM rectifier 10 and the inverter 2). The PWM control signal is generated so that the main circuit unit 11 of the PWM rectifier 10 generates the AC power with a power factor 1 and maintains the DC voltage value which is an output of the PWM rectifier 10 at a desired value. The PWM control signal is applied to the switching element in the main circuit unit 11 of the PWM rectifier 10.
FIG. 11 is a block diagram for describing a configuration of the PWM rectifier control unit illustrated in FIG. 10. The PWM rectifier control unit 12 includes a DC voltage loop control unit 31, a power supply phase calculation unit 32, a three-phase DQ conversion unit 33, a current loop control unit 34, a DQ three-phase conversion unit 35, and a PWM modulation unit 36.
On the basis of the DC voltage value detected by the DC voltage detection unit 23 and a DC voltage command which is input, the DC voltage loop control unit 31 generates a current command which causes the DC voltage value to match the DC voltage command. Note that a fixed value is generally used as the DC voltage command in the PWM rectifier 10. A power supply phase is calculated by the power supply phase calculation unit 32 from the AC voltage value detected by the AC voltage detection unit 21. By using the power supply phase, the three-phase DQ conversion unit 33 converts the three-phase AC current value detected by the AC current detection unit 22 into a current value on a DQ coordinate plane (hereinafter, referred to as “DQ phase current value”). The current loop control unit 34 generates a voltage command on the DQ coordinate plane (hereinafter, referred to as “DQ phase voltage command”) which causes the DQ phase current value to match the current command. The DQ three-phase conversion unit 35 converts the DQ phase voltage command into a three-phase voltage command using the power supply phase. The PWM modulation unit 36 compares the three-phase voltage command with a triangular-wave carrier signal having a predetermined carrier frequency to generate the PWM control signal for controlling a switching operation of a semiconductor switching element in the main circuit unit 11 of the PWM rectifier 10. According to such configuration, in the main circuit unit 11 of the PWM rectifier 10, the switching operation of the internal switching element is controlled by the PWM control signal to perform the power running or regenerative operation.
FIG. 12 is a block diagram for describing a configuration of the DC voltage loop control unit illustrated in FIG. 11. The DC voltage loop control unit 31 includes a subtractor 41, a PI control unit 42, and a current command restriction unit 43. In general, an upper limit value is provided for an absolute value of the current command generated by the DC voltage loop control unit 31 on the basis of a rating current of an element such as a switching element in the PWM rectifier 11. The current command restriction unit 43 clamps the current command at the upper limit value when a magnitude of the current command is equal to or greater than the upper limit value. In addition, when it is desired to reduce a power supply capacity, for example, a limit value of the current command in the current command restriction unit 43 may be set to be an even lower value based on the power supply capacity. Hereinafter, a state in which the current command has reached the limit value is referred to as a “saturation state of the DC voltage loop control unit”. Since the current command is held at at the limit value when the DC voltage loop control unit is in the saturation state, DC power with constant amplitude is always output from the PWM rectifier 10 and it is difficult to cause the DC voltage to follow the DC voltage command. Therefore, the DC voltage value decreases when the PWM rectifier 10 is in a power running state, and the DC voltage value increases when the rectifier is in a regenerative state.
The proportional-integral control (PI control) is performed on a difference between the DC voltage command and the DC voltage value, which is calculated by the subtractor 41, by the PI control unit 42 to generate the current command. When the magnitude of the current command is equal to or greater than the upper limit value, the current command restriction unit 43 clamps the current command at the upper limit value.
With the motor drive unit 100 including the above-mentioned PWM rectifier 10, the DC voltage command of the PWM rectifier 10 is lowered in advance when the motor 3 is in power running, so that the regenerative energy generated at the time of deceleration of the motor 3 is stored in the electric storage device 6 without returning the energy to the AC power supply side to use the energy at the next power running of the motor 3, whereby improving efficiency and reducing the power supply capacity.
For example, there is a method which suppresses a peak of AC power supplied from an AC power supply by controlling a current of a PWM rectifier under a predetermined current limit value as described in the Japanese Laid-open Patent Publication No. 2000-236679.
FIG. 13 is a timing chart schematically illustrating an operation of a motor drive unit described in the Japanese Laid-open Patent Publication No. 2000-236679. In this example, according to the method described in the Japanese Laid-open Patent Publication No. 2000-236679, a case is described in which the motor drive unit 100 illustrated in FIG. 10 to FIG. 12 is operated to cause the motor 3 to stop, accelerate, run at a constant speed, decelerate, and stop in this order. Note that FIG. 13 illustrates a “motor speed”, a “motor output”, a “rectifier output”, and “DC voltage command and DC voltage value” from the top. In addition, a dashed line illustrates the DC voltage command.
First, during a motor stop condition from time t0 to time t1, the PWM rectifier control unit 12 of the PWM rectifier 10 controls the DC voltage value of the electric storage device 6 to match the DC voltage command.
When staring acceleration of the motor 3 at time t1, entire energy required for the acceleration of the motor 3 is supplied from the AC power supply 4 side through the PWM rectifier 10 until the time t2. After that, when the energy required for the acceleration of the motor 3 reaches a limit value at time t2, it is less satisfied only with the energy supplied from the AC power supply 4 side through the PWM rectifier 10, and therefore, at time t2, the energy supply from the electric storage device 6 to the motor 3 is started. Therefore, the DC voltage value (electric potential of the electric storage device 6) decreases from time t2 to time t3. The limit value is set to such a value that the output of the PWM rectifier 10 is a value less than a motor maximum output.
When the motor 3 stops the acceleration and operates at a constant speed at time t3, the energy required for driving the motor 3 is less than the energy output from the PWM rectifier 10, and therefore from time t3, the DC voltage value (electric potential of the electric storage device 6) turns to increase. Then, the DC voltage value returns to the value of the DC voltage command at time t4. From time t4 to time t5, the DC voltage value follows the DC current command and keeps constant, and the motor drive unit becomes in a condition in which entire energy required for the operation of the motor 3 at the constant speed is supplied from the AC power supply 4 side through the PWM rectifier 10.
When the motor 3 starts deceleration at time t5, the regenerative energy is returned to the inverter 2 from the motor 3. The inverter 2 inverts the regenerative energy into DC power to return the DC power to the DC link side. Although the PWM rectifier 10 also performs a regenerative operation at this time, the absolute value of the DC power resulting from the regenerative energy is greater than the absolute value of the limit value for the output of the PWM rectifier 10, and the DC power is therefore stored in the electric storage device 6, which results in an increase of the DC voltage value (electric potential of the electric storage device 6). When the absolute value of the regenerative power from the motor 3 is smaller than the absolute value of the limit value for the output of the PWM rectifier 10 at time t6, the DC voltage value (electric potential of the electric storage device 6) decreases from time t6 to time t7.
Even when the motor 3 stops at time t7, the DC voltage value (electric potential of the electric storage device 6) has not reached the value of the DC voltage command, and therefore, the PWM rectifier 10 returns the energy to the AC power supply 4 side until the DC voltage value (electric potential of the electric storage device 6) reaches the value of the DC voltage command. Then, the DC voltage value returns to the value of the DC voltage command at time 8. After the time t8, the PWM rectifier 10 controls the DC voltage value to match the DC voltage command.
In addition, for example, there is a method which suppresses AC power supplied from an AC power supply by gradually decreasing a DC voltage command at the time of acceleration of a motor and gradually increasing the DC voltage command at the time of deceleration of the motor, so that the regenerative energy of the motor is stored in an electric storage device, as described in the Japanese Laid-open Patent Publication No. 2012-085512.
In addition, for example, there is a method which suppresses AC power supplied from an AC power supply by setting the optimal DC voltage command for each operation pattern of a motor, so that the regenerative energy of the motor is stored in an electric storage device, as described in the Japanese Laid-open Patent Publication No. 2010-260094.
FIG. 14 is a timing chart schematically illustrating an operation of the motor drive unit described in the Japanese Laid-open Patent Publication No. 2012-085512 and the Japanese Laid-open Patent Publication No. 2010-260094. In this example, according to the method described in the Japanese Laid-open Patent Publication No. 2012-085512 or and Japanese Laid-open Patent Publication No. 2010-260094, a case is described in which the motor drive unit 100 illustrated in FIG. 10 to FIG. 12 is operated to cause the motor 3 to stop, accelerate, run at a constant speed, decelerate, and stop in this order. Note that FIG. 14 illustrates a “motor speed”, a “motor output”, a “rectifier output”, and “DC voltage command and DC voltage value” from the top. In addition, a dashed line illustrates the DC voltage command.
In an operation pattern in which the motor 3 is stopped from time t0 to time t1, is accelerated from time t1 to time t3, is operated at a constant speed from time t3 to time t4, is decelerated from time t4 to time t5, and is stopped from time t5 to time t7, a case will be described in which the DC voltage command is set as illustrated in the drawing so that the regenerative energy of the motor 3 is stored in the electric storage device, for one example. Specifically, the energy stored in the electric storage device 6 is made to discharge to supply the energy to the motor 3 by decreasing the DC voltage command of the PWM rectifier 10 at the time of acceleration of the motor 3. The regenerative energy generated by the motor 3 is made to be stored in the electric storage device 6 without returning the energy to the power supply by increasing the DC voltage command of the PWM rectifier 10 at the time of deceleration of the motor 3.
First, during a motor stop condition from time t0 to time t1, the PWM rectifier control unit 12 of the PWM rectifier 10 controls the DC voltage value of the electric storage device 6 to match the DC command.
When starting acceleration of the motor 3 at time t1, entire energy required for the acceleration of the motor 3 is supplied from the AC power supply 4 side through the PWM rectifier 10 until the time t2. After that, when the energy required for the acceleration of the motor 3 reaches a limit value at time t2, it is less satisfied only with the energy supplied from the AC power supply 4 side through the PWM rectifier 10, and therefore, at time t2, the PWM rectifier 10 is controlled to decrease the DC voltage command, so that the energy is also supplied from the electric storage device 6 to the motor 3. Therefore, the DC voltage value (electric potential of the electric storage device 6) decreases according to the decrease of the DC voltage command from time t2 to time t3.
When the motor 3 stops the acceleration and operates at a constant speed at time t3, the energy required for driving the motor 3 is less than the energy output from the PWM rectifier 10. In the illustrated example, in order to effectively utilize the energy stored in the electric storage device 6, the DC voltage command is reduced at a smaller ratio than a case of the motor deceleration from time t2 to time t3, so that the energy supply from the electric storage device 6 to the motor 3 is continued. Therefore, from time t3 to time t4, the energy required for the operation of the motor 3 at the constant speed is sufficiently supplied by the energy supplied from the AC power supply 4 side through the PWM rectifier 10 and the energy from the electric storage device 6.
When the motor 3 starts deceleration at time t4, the regenerative energy is returned to the inverter 2 from the motor 3. The inverter 2 inverts the regenerative energy into DC power to return the DC power to the DC link side. At this time, by controlling a power conversion operation of the PWM rectifier 10 to be stopped and the DC voltage command to be increased, the DC power resulting from the regenerative energy is stored in the electric storage device 6. Therefore, from time t4 to time t5, the DC power is stored in the electric storage device 6, so that the DC voltage value (electric potential of the electric storage device 6) increases.
When the motor 3 stops at time t5, the PWM rectifier 10 controls the DC voltage value to match the DC voltage command kept constant. Since the energy stored in the electric storage device 6 is consumed by an internal resistance of the PWM rectifier 10 or the like, the PWM rectifier 10 controls the DC voltage value to match the DC voltage command, so that the AC power from the AC power supply 4 side is converted into DC power to supply the energy sufficient for compensating the consumption.
When addressing the electric power peak which occurs at the time of accelerating or decelerating the motor as described above, the power supply capacity on AC power supply side tends to become large.
According to the invention described in the Japanese Laid-open Patent Publication No. 2000-236679, providing an upper limit for the output of a PWM rectifier enables a suppression of the peak of the energy supplied from the PWM rectifier. However, from a fact that the DC voltage command is always constant, the PWM rectifier continues to supply the energy from the AC power supply side to the DC link even after finishing the acceleration of a motor. Therefore, an electric storage device is charged to the condition before the motor acceleration, and whereby the regenerative energy generated at the time of motor deceleration cannot be stored in the electric storage device. Accordingly, the regenerative energy is required to return to the AC power supply side or to be consumed by an electric discharge resistor (not illustrated) in a DC link. In this way, according to the invention described in the Japanese Laid-open Patent Publication No. 2000-236679, the regenerative power generated at the time of motor deceleration cannot be effectively reused in the subsequent timing of motor start, and the power supply capacity is reduced not much.
In addition, according to the invention described in the Japanese Laid-open Patent Publication No. 2012-085512 and the Japanese Laid-open Patent Publication No. 2010-260094, regenerative energy generated by a motor is stored in an electric storage device without returning the energy to a power supply by increasing a DC voltage command of a PWM rectifier at the time of motor deceleration, and the energy stored in the electric storage device is made to discharge to reuse for acceleration of the motor by decreasing the DC voltage command of the PWM rectifier at the time of motor acceleration. Thereby, the electric power peak which occurs in the motor acceleration and deceleration is suppressed, and the power supply capacity on the AC power supply side is reduced. However, the DC voltage command is required to be set by trial and error for each operation pattern of a motor, which is not practical.